1. Field of the Invention
The present invention concerns a double-walled tube type open rack evaporating device and, more in particular, it relates to a double-walled tube type open rack evaporating device for evaporating a liquefied natural gas by using a heat medium.
2. Description of the Prior Art
As an open rack type evaporating device for evaporating a liquefied natural gas (hereinafter simply referred to as LNG) by using a heat medium such as sea water, a single-tube open rack type evaporating device has been used predominantly so far.
A single type open rack type evaporating device has a structure as shown in FIG. 12, in which a heat-exchanging panel 1 is formed by disposing a plurality of single tube heat-exchanging pipes in parallel. LNG is supplied from a lower header tank 2 disposed to the lower end of the panel 1. As LNG uprises in the pipes, it is evaporated under heating with sea water as a heat medium sprayed from troughs 3, 3 disposed on both sides above the heat-exchanging panel and then supplied externally as a natural gas (hereinafter referred to as NG) from an upper header tank 4 disposed to the upper end of the heat-exchanging panel 1.
The single tube open rack type evaporating device has a simple structure as described above and can be manufactured with ease. However, since heat-exchange is conducted directly between sea water as the heat medium and LNG at a cryogenic temperature through walls of the single tubes of the heat-exchanging panel 1, ice is deposited and increased to the outside of the pipes, that is, at the surface of the heat-exchanging panel 1.
Referring more specifically, if the flow rate of water supplied by spraying to each of the heat exchanging pipes of the heat-exchanging panel is localized, the height of ice deposited to the outer surface of the pipes becomes irregular. The degree of irregularity of the ice deposition is increased along with the increase of the LNG flow rate.
As a result, the temperature for each of the heat-exchanging pipes themselves becomes not uniform to cause difference in the shrinkage in each of the heat-exchanging pipes, to increase the thermal stresses, so that improvement for the performance as the evaporating device greatly suffers from restriction.
In view of the above, for preventing such irregular ice deposition to the surface of the heat-exchanging panel 1, it has been proposed a means, for example, of making the water spray amount uniform for each of the heat-exchanging pipes or moderating thermal stresses, but the fundamental drawback of the single tube type open rack evaporating device can not yet be overcome.
As a method of dissolving the problem, although it may be considered such a constitution of using a double-walled tube type structure for the heat-exchanging pipes to heat LNG indirectly, this brings about various problems, for example, the heat conduction efficiency is poor and the temperature of NG is lowered by LNG at a cryogenic temperature and supply of a gas at a normal temperature needs heating of a low temperature gas after leaving the evaporating device. No optimum constitution for an LNG evaporating device has yet been proposed in view of the selection for the position of disposing header tanks and troughs, method of utilizing a passage between the inner tube and the outer tube, considering the heat conduction efficiency.